In recent years, as the effort to solve global environmental issues, a variety of energy storage devices such as lithium ion secondary batteries are widely used, for example, the shift from gasoline automobiles to hybrid automobiles and electric vehicles has been promoted and the use of electric bicycles has been increased. Therefore, in such an energy storage device, increases of a power and a capacity of the device are increasingly required. Thus, conventionally, an energy storage device which increases its power and capacity by reducing a thickness of a separator is proposed (for example, refer to JP-A-2006-32246).
However, in the conventional energy storage device in which the thickness of the separator is reduced, a power may be temporarily reduced when performing charge-discharge. Particularly, when charge-discharge is performed repeatedly at a high-rate cycle, in the conventional energy storage device, a power may be temporarily reduced to a large extent. Such a temporary reduction in power (hereinafter, referred to as a transient power degradation) after the high-rate cycle can be improved by switching charge-discharge of the energy storage device to a low-rate cycle or avoiding charging for a certain period of time, but it is necessary to change to operating conditions other than charge-discharging conditions in a high-rate cycle.